Abstract Older adults are the most inactive, sedentary population group at the greatest risk for health conditions related to sedentary behavior (SB) and insufficient physical activity (PA). As older women significantly out- number men in later life and disproportionately account for aging-related health care costs, feasible and effective strategies for sustaining health and physical functioning in aging women are needed. Project 1 of the Sedentary Time and Aging Research (STAR) Program will investigate the acute metabolic and physiological impact of 3 different sitting interruption modalities in a controlled laboratory setting. Project 1 outcomes are related to healthy aging and significant for overweight/obese postmenopausal women, the study population target. A few laboratory studies, focused primarily on young, healthy adults, have examined different doses and frequencies of PA interruptions of prolonged sitting, 2 published studies interrupt sitting with standing, and none has included adults older than 75 years. Most sitting interruptions tested to date are not acceptable for older adults in the real world. Project 1 will study sitting interruption modalities that we know are feasible in older adults, based on our pilot studies. Several laboratory studies show that PA breaks in prolonged sitting improve postprandial metabolic regulation and prevent prolonged sitting-induced endothelial function impairment. Thus, our proposed study will concurrently assess the acute metabolic and endothelial function outcomes associated with PA, sit-to-stand, and standing break interruptions of prolonged sitting. Physiologic evidence suggests that these sitting interruption modalities will increase muscle contractions and blood flow to extremities, leading to improvement in our outcome measures. We hypothesize that acute interruptions of prolonged sitting will improve parameters of metabolism and endothelial function associated with healthy aging. We will test this hypothesis using a 4-period, 4-condition randomized crossover clinical laboratory study in 86 overweight/obese postmenopausal women. The 4 conditions, conducted over a 5-hr period of sitting on 4 separate days, are: A) prolonged sitting (Control), B) brief sit-to- stand interruptions every 20 min (Frequent Sit-to-Stand Transitions), C) 10-min stands every hour (Stand More), and D) brief PA interruptions every hour (PA Breaks). We are currently conducting a pilot study of equivalent design with recruitment success and ongoing outcome analyses. Project 1 will compare postprandial dynamics of plasma insulin and glucose and blood pressure sampled at 30-min intervals across the 4 conditions and will compare condition-specific changes in endothelial function across each day using femoral flow-mediated dilation. Project 1 will investigate acute outcomes, complementing the 3-mo and 5- to 7- yr outcomes of Projects 2 & 3. Project 1 will provide data to refine the computational techniques applied to Project 3. The novel set of conditions and concurrently measured outcomes in Project 1 will improve evidence on the acute impact of sitting interruption types on healthy aging.